what is the "density" exactly means?

[Peregrinus]

3rd density will not be here when the social memory complex is formed in 4D, as I believe Ra stated that it would be half way through 4D when the complex is typically formed, and the length is

43.13 Questioner: How long is a cycle of experience in fourth density in our years?

Ra: The cycle of experience is approximately 30 million of your years if the entities are not capable of being harvested sooner. There is in this density a harvest which is completely the function of the readiness of the social memory complex. It is not structured as is your own, for it deals with a more transparent distortion of the One Infinite Creator.

There is on this planet between 400 to 500 years of 3rd density light left.